How to add a new matter for physics in blender ? (aluminium, titanium etc.)
Extra menu script.
AFAIK there is no simple way to tack extra material / density pairs to the operator enum. It is however quite simple, via scripting,  to add a menu of extra options.

Quick slap together proof of concept using Templates &gt; Python &gt; UI Menu Simple  Material &quot;Foo&quot; has density 12.0, &quot;Bar&quot; 0.01,  etc.
import bpy

extra_mats = (
    (&quot;Foo&quot;, 12.0),
    (&quot;Bar&quot;, 0.01),
    (&quot;Goo&quot;, 11),
    )
class SimpleCustomMenu(bpy.types.Menu):
    bl_label = &quot;Extras&quot;
    bl_idname = &quot;OBJECT_MT_simple_custom_menu&quot;

    def draw(self, context):
        layout = self.layout
        for name, p in extra_mats:
            op = layout.operator(
                &quot;rigidbody.mass_calculate&quot;, 
                text=f&quot;{name} ({p})&quot;
                )
            op.density = p
            op.material = &quot;Custom&quot;
        

def draw_menu(self, context):
    row = self.layout.row()
    row.operator(&quot;rigidbody.mass_calculate&quot;)
    row.menu(SimpleCustomMenu.bl_idname)
    
def register():
    bpy.utils.register_class(SimpleCustomMenu)
    bpy.types.PHYSICS_PT_rigid_body_settings.prepend(draw_menu)


def unregister():
    bpy.utils.unregister_class(SimpleCustomMenu)
    bpy.types.PHYSICS_PT_rigid_body_settings.remove(draw_menu)

if __name__ == &quot;__main__&quot;:
    register()

    # The menu can also be called from scripts
    bpy.ops.wm.call_menu(name=SimpleCustomMenu.bl_idname)